The present invention relates in general to a blower element of a heating, ventilating, air conditioning (HVAC) system for automobiles, and, more specifically, to blower inlet structures that reduce tonal noise generation without reducing the quantity of air flow.
In a typical automotive HVAC system, a blower delivers fresh or recirculated air to heat exchangers (e.g., an evaporator or a heater core) which is then distributed to the passenger cabin via ducts and outlet registers. The blower includes a housing that contains a fan (i.e., impeller) and a motor for rotating the fan. One of the most common configurations for an automotive HVAC air handling system uses a centrifugal blower, wherein a cylindrical arrangement of fan blades receives inlet air via an axial opening in the housing and centrifugally accelerates the air through a surrounding scroll region to an outlet. The scroll region has a volute shape in which the cross-sectional area of the scroll increases as the outlet is approached.
A centrifugal impeller typically is formed as a rotating blower wheel having a plurality of cylindrically-placed fan blades. An inlet arranged over one side of the blower wheel has a throat with an inside diameter slightly less that the inside diameter between the fan blades so that air is directed to the inside axis of the blower wheel to be accelerated into the outer scroll region. The throat is usually ring-shaped having an inward arc and/or other features to create a venturi effect that increases air flow into the blower.
The typical throat arrangement tends to induce a strong air flow interaction with the moving blades near the ring. The interaction generates an audible tone occurring at a fundamental frequency determined by the number of fan blades multiplied by the speed of rotation, referred to as the Blade Passing Frequency (BPF). The BPF tone usually sounds like a high-pitched whistle. This noise can become objectionable to occupants of the vehicle.